Methods to make diffractive optical elements

ABSTRACT

A method for forming a diffractive lens includes forming an etch stop layer on a first surface of a silicon substrate, forming a diffractive optical element above the etch stop layer, forming a planarization layer covering the diffractive optical element, planarizing the planarization layer, forming a bonding layer on the planarization layer, bonding a transparent substrate on the bonding layer, and etching a second surface of the silicon substrate to the etch stop layer to remove a portion of the silicon substrate opposite the diffractive optical element, wherein the remaining portion of the silicon substrate forms a bonding ring.

FIELD OF INVENTION

This invention relates to a method for making a diffractive opticalelement (DOE) using complementary metal oxide semiconductor (CMOS)processes.

DESCRIPTION OF RELATED ART

Modern optical systems often make use of diffractive optical elements(DOEs). For example, a DOE lens can be used to focus a laser light intoan optical fiber. DOEs operate by modifying the phase of lightinteracting with the element either in transmission or by reflection.DOEs are formed by patterning phase shifting materials into anappropriate lens.

SUMMARY

In one embodiment of the invention, a method for forming a diffractivelens includes forming an etch stop layer on a first surface of a siliconsubstrate, forming a diffractive optical element above the etch stoplayer, forming a planarization layer covering the diffractive opticalelement, planarizing the planarization layer, forming a bonding layer onthe planarization layer, bonding a transparent substrate on the bondinglayer, and etching a second surface of the silicon substrate to the etchstop layer to remove a portion of the silicon substrate opposite thediffractive optical element, wherein the remaining portion of thesilicon substrate forms a bonding ring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method for making a diffractive opticalelement (DOE) on a transparent substrate in one embodiment of theinvention.

FIGS. 2A, 2B, 2C, 2D, and 2E illustrate the resulting structures formedusing the method of FIG. 1 in one embodiment of the invention.

FIG. 3 is a flowchart of a method for making a DOE on a transparentsubstrate in another embodiment of the invention.

FIG. 4 illustrates the resulting structure formed using the method ofFIG. 3 in one embodiment of the invention.

FIG. 5 is a flowchart of a method for making a DOE on asilicon-on-insulator (SOI) substrate in another embodiment of theinvention.

FIGS. 6A, 6B, 6C, 6D, 6E, 6F, and 6G illustrate the resulting structuresformed using the method of FIG. 5 in one embodiment of the invention.

FIG. 7 is a flowchart of a method for making a DOE on a siliconsubstrate and transferring the DOE to a transparent substrate in anotherembodiment of the invention.

FIGS. 8A, 8B, 8C, 8D, 8E, 8F, and 8G illustrate the resulting structuresformed using the method of FIG. 7 in one embodiment of the invention.

FIG. 9 is a flowchart of a method for making a DOE by transferring apattern of the DOE from a silicon substrate to a transparent substratein another embodiment of the invention.

FIGS. 10A, 10B, 10C, and 10D illustrate the resulting structures formedusing the method of FIG. 9 in one embodiment of the invention.

Use of the same reference symbols in different figures indicates similaror identical items. The cross-sectional figures are not drawn to scaleand are only for illustrative purposes.

DETAILED DESCRIPTION

A diffractive optical element (DOE) can be fabricated by formingmultilayer materials of amorphous silicon and silicon dioxide supportedon a silicon substrate and successively patterning the multilayermaterials to form the DOE. Such a DOE takes advantage of the highrefractive index of the silicon, the alternating etch stop layers in themultilayer materials, and the precision masking offered by complementarymetal oxide semiconductor (CMOS) fabs. Such a DOE is well suited forlong wavelengths where the silicon substrate is transparent. Due to highabsorption of the silicon substrate in the near infrared for wavelengthsbelow 1 micron (μm), such a DOE cannot be applied to shorter wavelengthssuch as 850 and 990 nanometer (nm) generated by some lasers.

FIG. 1 is a flowchart of a method 10 to form a DOE on a transparentwafer in one embodiment of the invention. Method 10 is hereafterexplained in reference to FIGS. 2A to 2E.

In step 12, as shown in FIG. 2A, an opaque coating 32 is formed on thetopside of a transparent substrate 34. Opaque coating 32 is applied totransparent substrate 34 so semiconductor equipment using opticalsensors can detect and handle transparent substrate 34. Opaque coating32 is only shown in FIG. 2A. Transparent substrate 34 is transmissive toa wavelength of interest (e.g., selected from infrared to ultraviolet)if a percentage of the incident light specified by the application istransmitted through the substrate (e.g., at least 10% for mostcommunication applications). Transparent substrate 34 can be quartz,sodium borosilicate glass (e.g., Pyrex®), sapphire, or fuse silica.Opaque coating 32 can be amorphous silicon (α—Si) deposited by lowpressure chemical vapor deposition (LPCVD) or plasma enhanced chemicalvapor deposition (PECVD). Amorphous silicon coating 32 is typically morethan 0.5 μm thick.

In step 14, as shown in FIG. 2A, an antireflective (AR) coating 36 isoptionally formed on the backside of transparent substrate 34. ARcoating 36 reduces reflection as light travels between a DOE 50 (FIG.2B) and transparent substrate 34.

Those skilled in the art understand the choice of the AR coatingmaterial depends on the refractive index of the incoming medium, theexit medium, and the AR coating material. The ideal AR coating materialhas a refractive index that is the geometric mean of the incoming mediumand the exit medium. The ideal AR coating thickness is equal to aquarter of the wavelength of the light in the AR coating medium. In oneembodiment, AR coating 36 is sandwiched between Pyrex® and silicon.Hence, the ideal refractive index is calculated as follows: sqrt(1.5*3.5)=2.29, where 1.5 and 3.5 are the refractive indices of Pyrex®and silicon respectively. Thus, titanium dioxide (TiO₂) is selected asthe AR material because it has a reflective index of 2.2 that is closeto the ideal value of 2.29. The thickness of the TiO₂ AR coating 36 for990 nm operation is then calculated as follows: 990 nm/4/2.2=112.5 nm.Those skilled in the art understand that the refractive index and thethickness of AR coating 36 can be adjusted accordingly. TiO₂ AR coating36 can be deposited by electron beam (e-beam) evaporation.

In step 16, as shown in FIG. 2B, a DOE 50 is formed on AR coating 36.DOE 50 includes a stack of phase shifting layers patterned to form thedesired diffractive lens. Adjacent phase shifting layers in the stackare separated by an etch stop layer. The phase shifting layers can beamorphous silicon (α—Si) and the etch stop layers can be silicon dioxide(SiO₂). Alternatively, the phase shifting layers can be silicon nitride(Si₃N₄) instead of amorphous silicon.

The DOE layers can be grown by PECVD. The thicknesses of the amorphoussilicon and the oxide layers depend on the wavelength of interest, thenumber of the layers, and the refractive index of the materials. Thetotal phase shift for light through the DOE stack compared to phaseshift going through the air should be an integral multiple of 2 π. Totake advantage of the high refractive index of silicon, the lower indexoxide layer is minimized to about 50 Å. This thickness wasexperimentally determined to be an effective etch stop layer. Hence, fora typical DOE with 8 interleaving layers of amorphous silicon and oxidelayer operating at 990 nm, the total phase shift due to the eight oxideetch stop layers is calculated as follows: $\begin{matrix}{{\left( {{RIox} - {RIAir}} \right)*2\pi*\left( {5\quad{{nm}/990}\quad{nm}} \right)*8} = {\left( {1.45 - 1} \right)*\quad 2\pi*\left( {5\quad{{nm}/990}\quad{nm}} \right)*8}} \\{= {0.1142.}}\end{matrix}$RIox and RIAir are the refractive indices of oxide etch stop layer andair, respectively. The values 1.45 and 1 are the refractive indices ofthe oxide layer and the air, respectively. The total phase shift due tothe eight amorphous silicon layer is:(RIsi−RIAir)*2 π*(t/990 nm)*8.RIsi is the refractive index of the amorphous silicon layer and t is thethickness of that layer. To achieve the desired phase shift through theDOE stack, each of the amorphous silicon layer thickness is calculatedas follows:2 π−0.1142=(RIsi−RIAir)*2 π*(t/990 nm)*8, ort=(2 π−0.1142)*990 nm/[(RIsi−RIAir)*2 π*8]t=(2 π−0.1142)*990 nm/[(3.5−1)*2 π*8]t=48.6 nm.The value 3.5 is the refractive index of the amorphous silicon layer.

Once the stack is formed, the top amorphous silicon layer is masked byphotoresist and then etched using the oxide layer to stop the etching.After the silicon is etched, the etch chemistry is changed and the oxidelayer is etched using the next silicon layer to stop the etching. Eachsuccessive layer is masked and etched to form the desired diffractivelens.

In one embodiment, DOE 50 is a bifocal diffractive lens that convertslaser light into a small angle distribution that is spread uniformlythroughout a volume. The volume's dimensions are large relative to thesize of the input face of an optical fiber so the components can easilyalign. The bifocal diffractive lens has a surface with ridges thatprovide two focal lengths f1 and f2. A design process for the bifocaldiffractive lens can begin with determining a first phase function thatdefines a surface contour for a conventional diffractive lens havingfocal length f1. Any conventional techniques for diffractive lens designcan be used. In particular, commercial software such as GLAD fromApplied Optics Research, Inc. or DIFFRACT from MM Research, Inc. cananalyze the phase functions of diffractive elements. A second phasefunction is similarly generated, wherein the second phase function issuch that if the second phase function were multiplexed together withthe first phase function, the combination would provide a diffractivelens having the second focal length f2. The second phase function isthen scaled so as to provide a partially efficient diffractive lens thatfocuses a percentage (e.g., 50%) of the incident light but passes theremainder (e.g., 50%) of the incident light unperturbed. The first phasefunction and the scaled second phase function are multiplexed togetherto form a final bifocal lens design.

In another embodiment, DOE 50 is a hybrid diffractive/refractiveelement. The hybrid diffractive/refractive element spreads the lightover a volume to expand the alignment tolerance for an optical fiber asdescribed above. The hybrid diffractive/refractive lens has at least onesurface with a curvature for one focal length, e.g., f2. Further,diffractive features of a partially efficient diffractive lens aresuperimposed on one or both surfaces of the hybriddiffractive/refractive lens so that the combination provides two focallengths f1 and f2 for separate fractions of the incidence light.

In step 18, as shown in FIG. 2B, an AR coating 51 is optionally formedover DOE 50. The refractive index and thickness of AR coating material51 can be selected as described above. AR coating 51 can be siliconnitride (Si₃N₄). Thin film silicon nitride material can have a range ofrefractive index depending on deposition conditions. Nitride AR coating51, with a refractive index of 1.9, can be deposited by e-beamevaporation and is typically 130 nm thick for 990 nm wavelength. The useof e-beam evaporation provides a non-conformal coating which is moredesirable on the grating surface. AR coating 51 is only illustrated inFIG. 2B.

In step 20, as shown in FIGS. 2C and 2D, a silicon substrate 62 isprocessed to form a bonding ring 62A. A barrier layer 60 is formed onsilicon substrate 62. Barrier layer 60 separates any metal from reactingwith silicon substrate 62 so that metal silicide will not form in hightemperature operations later on. Barrier layer 60 can be silicon nitride(Si₃N₄). Nitride barrier layer 60 can be deposited by PECVD and istypically 0.5 μm thick. Bonding pads 64 are then formed on barrier layer60. Bonding pads 64 can be formed by patterning a liftoff mask,depositing a metal, and lifting off the mask with the metal. Metalbonding pads 64 can be titanium-platinum-gold (TiPtAu) sequencedeposited by e-beam evaporation or sputtering. Titanium has a typicalthickness of 50 nm, platinum has a typical thickness of 150 nm, and goldhas a typical thickness of 50 nm. A photoresist 66 is then spun,exposed, and developed on barrier layer 60 to form an etch window 67.Portions of barrier layer 60 and silicon substrate 62 exposed by etchwindow 67 are etched away to form a bonding ring 62A. Barrier layer 60and silicon substrate 62 can be etched using deep reactive ion etching(DRIE). Photoresist 66 is then stripped away.

In step 22, as shown in FIG. 2D, bonding ring 62A is bonded totransparent substrate 34 to form a lid 80. If transparent substrate 34is sodium borosilicate glass, bonding ring 62A can be bonded totransparent substrate 34 by an anodic bond. If transparent substrate 34is quartz or sapphire, an adhesive bond can be used. Alternatively, ahydrofluoric acid (HF) bond can also be used. In HF bonding, two cleansurfaces that are pressed together can be joined by dispensing a smallamount of HF between the two surfaces. The HF will fill in the gapbetween the joining pieces by capillary action. When the HF dries up,the two pieces will be joined permanently. Alternatively, a glass fritbond can be used. In glass frit bonding, a fine powder of glass isdispensed between the two joining pieces and heated above the glassreflow temperature during the joining process. This temperature dependson the selected glass frit material and is typically above 250° C.

In step 24, as shown in FIG. 2E, lid 80 is bonded to a submount 82 tocomplete a microelectronic package 84. Lid 80 and submount 82 can bebonded by solder. Submount 82 can include a light source 86 (e.g., avertical cavity surface emitting laser (VCSEL)). Submount 82 can alsoinclude other active and passive circuitry 88.

FIG. 3 is a flowchart of a method 90 to form a DOE on a transparentwafer in another embodiment of the invention. Method 90 follows steps 12to 18 of method 10 in FIG. 1. Step 18 is then followed by step 26 and ishereafter explained in reference to FIG. 4.

In step 26, as shown in FIG. 4, transparent substrate 34 with DOE 50 ismounted to a submount 92 with a silicone material 94. Submount 92 caninclude a light source 96 (e.g., a VCSEL). Submount 82 can also includeother active and passive circuitry 98.

FIG. 5 is a flowchart of a method 100 to form a DOE on asilicon-on-insulator (SOI) substrate in another embodiment of theinvention. Method 100 is hereafter explained in reference to FIGS. 6A to6G.

In step 104, as shown in FIG. 6A, DOE 50 is formed on an SOI substrate142. SOI substrate 142 includes a silicon device layer 144, an oxideinsulator layer 146, and a silicon handle layer 148. Silicon devicelayer 144 is typically less than 20 μm thick, which makes ittransmissive to wavelengths of 850 and 990 nm. Oxide insulator layer 146is later used as an etch stop when silicon handle layer 148 is etched toform a bonding ring 148A (FIG. 6E). In one embodiment, SOI substrate 142further includes a silicon nitride layer 145 (shown only in FIG. 6A)between device layer 144 and oxide insulator layer 146. Such a SOIsubstrate 142 can be formed by bonding an oxidized wafer with a waferthat is coated with silicon nitride. After the bonding, the wafer withsilicon nitride can be grind and lapped thin to form device layer 144.Using such a SOI substrate 142 would eliminate step 128 described later.

DOE 50 is formed as described above in method 10 of FIG. 1 but with thefollowing modification. In method 10, the phase shift is due todifferent speed of light travelling in DOE 50 and air. In thisembodiment, the phase shift is due to different speed of lighttravelling in DOE 50 and oxide planarization layer 160 (FIG. 6C). Sinceoxide planarization layer 160 has a refractive index equal or similar tothe etch stop oxide in DOE 50, the amorphous silicon thickness iscalculated as follows: (990 nm/[(8*(3.5−1.45)])=60.37 nm. The total DOEstack thickness is calculated as follows: [990 nm/(3.5−1.45)+5 nm]*8=523 nm.

In step 106, as shown in FIG. 6B, an AR coating 150 is optionally formedover DOE 50. The material and the thickness of AR coating 150 areselected as described above. AR coating 150 can be titanium dioxide(TiO₂). TiO₂ AR coating 150 can be deposited by e-beam evaporation andis typically 112.5 nm thick for 990 nm wavelength. AR coating 150 isonly illustrated in FIG. 6B.

In step 108, as shown in FIG. 6C, a planarization layer 160 is formedover device layer 144 and DOE 50, and then planarized if a flat topsurface on the resulting structure is desired. Planarization layer 160can be silicon dioxide (SiO₂) formed by PECVD. Oxide planarization layer160 is typically 0.3 μm thicker than DOE 50 so it can be polished bychemical mechanical polishing (CMP) to a flatness of better than 200angstroms (Å). In one embodiment for 990 nm wavelength, DOE 50 has atypical thickness of 523 nm. Thus, oxide planarization layer 160 has atypical thickness of 823 nm.

In step 110, as shown in FIG. 6D, a barrier layer 170 is formed on thebackside of SOI substrate 142. Barrier layer 170 can be silicon dioxide(SiO₂). Oxide barrier layer 170 can be deposited by PECVD and istypically 0.5 μm thick.

In step 112, as shown in FIG. 6D, bonding pads 172 are formed on barrierlayer 170. Bonding pads 172 can be titanium-platinum-gold (TiPtAu)sequence deposited by e-beam evaporation or sputtering and patterned bya liftoff mask. Titanium has a typical thickness of 50 nm, platinum hasa typical thickness of 150 nm, and gold has a typical thickness of 50nm.

In step 114, as shown in FIG. 6D, an etch mask layer 174 is formed overbarrier layer 170 and bonding pads 172. If a wet etch is to be used,etch mask layer 174 can be silicon nitride (Si₃N₄) deposited by PECVD.If a dry etch is to be used, etch mask layer 174 can be a photoresistthat is spun on.

In step 116, as shown in FIG. 6E, etch mask layer 174 is patterned toform part of an etch window 176. If etch mask layer 174 is nitride, itcan be patterned by spinning on a photoresist, exposing the photoresist,developing the photoresist, and etching the nitride. If etch mask layer174 is a photoresist, it can be patterned by exposing and developing thephotoresist.

In step 118, as shown in FIG. 6E, barrier layer 170 is patterned usingetch mask layer 174 to form part of etch window 176.

In step 120, as shown in FIG. 6E, a portion of silicon handle layer 148located opposite of DOE 50 and exposed by etch window 176 is etched downto oxide insulator layer 146, which acts as an etch stop to form abonding ring 148A. If etch mask layer 174 is nitride, then siliconhandle layer 148 can be etched with a potassium hydroxide (KOH)solution. If etch mask layer 174 is a photoresist, then silicon handlelayer 148 can be etched using DRIE.

In step 122, as shown in FIGS. 6E and 6F, the remaining etch mask layer174 is etched away.

In step 124, as shown in FIGS. 6E and 6F, a portion of oxide insulatorlayer 146 opposite DOE 50 is etched away. Oxide insulator layer 146 canbe etched away using hydrofluoric acid (HF).

In step 126, as shown in FIG. 6G, an AR coating 190 is optionally formedon planarization layer 160. The material and the thickness of AR coating190 are selected as described above. AR coating 190 can be magnesiumfluoride (MgF₂). MgF₂ AR coating 190 can be deposited by e-beamevaporation and is typically 179.3 nm thick for 990 nm wavelength.

In step 128, as shown in FIG. 6G, an AR coating 192 is optionally formedon silicon device layer 142 opposite DOE 50. The material and thicknessof AR coating 192 are selected as described above. AR coating 192 can besilicon nitride (Si₃N₄). Nitride AR coating 192 can be deposited bye-beam evaporation through a shadow mask and is typically 132.3 nm for990 nm wavelength. The shadow mask can be a chem-etched metal foil. Theevaporated silicon nitride from the source will pass through the holesin the mask and deposit onto the wafer. This mask is normallymechanically aligned and placed close to the wafer. Note that materialssuch as silicon nitride and amorphous silicon have a range of refractiveindices that can be adjusted by process parameter. Those skilled in theart understand how to adjust refractive index by adjusting parameterssuch as deposition temperature, gas flow conditions, and pressure. Atthis point, a lid 194 is formed and can be mounted to a submount tocomplete a microelectronic package.

Alternatively, step 128 can be bypassed if a SOI substrate 142 includinga silicon nitride layer 145 (FIG. 6A) is used. If such a SOI substrate142 is used, then silicon nitride layer 145 becomes the AR coating forDOE 50.

FIG. 7 is a flowchart of a method 200 to form a DOE on a siliconsubstrate and transferring the DOE onto a transparent substrate in oneembodiment of the invention. Method 200 is hereafter explained inreference to FIGS. 8A to 8G.

In step 202, as shown in FIG. 8A, an etch stop layer 240 is formed onthe topside of a silicon substrate 242. Etch stop layer 240 can besilicon dioxide (SiO₂). Etch stop layer 240 can be thermally grown ordeposited by PECVD and is typically 0.5 μm thick.

In step 204, as shown in FIG. 8A, an AR coating 244 is optionally formedon etch stop layer 240. The material and the thickness of AR coating 244are selected as described above. AR coating 244 can be silicon nitride(Si₃N₄). Nitride AR coating 244 can be deposited by PECVD and istypically 132.3 nm thick for 990 nm wavelength.

In step 208, as shown in FIG. 8A, DOE 50 is formed on AR coating 244.DOE 50 is formed as described above in method 10 of FIG. 1. Thethicknesses of DOE layers can be selected as described above.

In step 210, as shown in FIG. 8B, an AR coating 248 is optionally formedover DOE 50. The material and the thickness of AR coating 248 areselected as described above. AR coating 248 can be titanium dioxide(TiO₂). TiO2 AR coating 248 can be deposited by e-beam evaporation andis typically 112.5 nm thick for 990 nm wavelength. AR coating 248 isonly illustrated in FIG. 8B.

In step 212, as shown in FIG. 8C, a planarization layer 260 is formedover AR coating 244 and DOE 50, and then planarized. Planarization layer260 can be silicon dioxide (SiO₂) formed by PECVD. Oxide planarizationlayer 260 is typically 0.3 μm thicker than DOE 50 so it can be polishedby CMP to a flatness of better than 200 Å. Oxide planarization layer 260is typically 0.8 μm thick.

In step 214, as shown in FIG. 8C, a bonding layer 262 is formed onplanarization layer 260. Bonding layer 262 can be amorphous silicon(α—Si) deposited by PECVD. Silicon bonding layer 262 has a thickness ofhalf a wavelength of interest, or an integral multiple of half awavelength of interest. Silicon bonding layer 262 is typically 141.4 nmfor 990 nm wavelength. Silicon bonding layer 262 improves anodic bondingof an oxide coated silicon substrate to a sodium borosilicate glass(e.g., Pyrex®) transparent substrate 270 (FIG. 8D). This layer can beomitted for bonding methods other than anodic bonding.

In step 216, as shown in FIG. 8D, transparent substrate 270 is bonded tobonding layer 262. Transparent substrate 270 provides the mechanicalsupport for the remaining structure after silicon substrate 242 is lateretched away. Transparent substrate 270 is transmissive to a wavelengthof interest (e.g., selected from infrared to ultraviolet) if apercentage of the incident light specified by the application istransmitted through the substrate (e.g., at least 10% for mostcommunication applications). Transparent substrate 270 can be quartz,sodium borosilicate glass (e.g., Pyrex®), sapphire, or fuse silica. Thepreferred transparent substrate 270 is sodium borosilicate glass thatcan bonded to bonding layer 262 by an anodic bond. Alternatively, anadhesive bond, a HF bond, or a glass frit bond can be used.

In step 218, as shown in FIG. 8E, a barrier layer 271 is formed on thebackside of silicon substrate 242. Barrier layer 271 can be silicondioxide (SiO₂). Oxide barrier layer 271 can be formed by PECVD and istypically 0.5 μm thick.

In step 220, as shown in FIG. 8E, bonding pads 272 are formed on barrierlayer 271. Bonding pads 272 can be titanium-platinum-gold (TiPtAu)sequence deposited by e-beam evaporation or sputtering and patterned bya liftoff mask. Titanium has a typical thickness of 50 nm, platinum hasa typical thickness of 150 nm, and gold has a typical thickness of 50nm.

In step 224, as shown in FIGS. 8E and 8F, a photoresist layer 274 isformed and patterned to form part of an etch window 278.

In step 226, as shown in FIGS. 8E and 8F, a portion of barrier layer 271exposed by etch window 278 is etched using photoresist layer 274 to formpart of etch window 278.

In step 228, as shown in FIGS. 8E and 8F, a portion of silicon substrate242 located opposite of DOE 50 and exposed by etch window 278 is etcheddown to etch stop layer 240 to form a bonding ring 242A. Siliconsubstrate 242 can be etched by DRIE.

In step 230, as shown in FIGS. 8F and 8G, the remaining photoresistlayer 274 is removed chemically by a resist striper.

In step 232, as shown in FIGS. 8F and 8G, a portion of oxide etch stoplayer 240 opposite DOE 50 is etched away to expose a portion of ARcoating 244. An optional AR coating of magnesium fluoride (MgF₂) can bee-beam evaporated onto transparent substrate 270. The thickness andrefractive index of the MgF₂ can be selected as described above. At thispoint, a lid 280 is formed and can be mounted to a submount to completea microelectronic package.

As an alternative to steps 224 to 232, all of silicon substrate 242 canbe etched away to form DOE 50 mounted to transparent substrate 270. Thisresults in a lid 280 without bonding ring 242A.

FIG. 9 is a flowchart of a method 360 to form a DOE by transferring apattern for the DOE from a silicon substrate to a transparent substratein one embodiment of the invention. Method 360 is hereafter explained inreference to FIGS. 10A to 10D.

In step 362, as shown in FIG. 10A, a mold 390 of a DOE is formed on asilicon substrate 392. Mold 390 can be formed similarly as DOE 50 inmethod 10 of FIG. 1 except it is an inverted image of the DOE instead ofthe DOE itself. Since the inverted image will be filled with siliconnitride with refractive index of 2.1, the total thickness of this mold390 will be 990 nm/(RIsin−Riair)=990 nm/(2.1−1)=900 nm.

In step 363, as shown in FIG. 10A, a non-conformal silicon dioxide(SiO₂) layer 393 is formed over mold 390. Non-conformal oxide layer 393can be deposited by e-beam evaporation. Oxide layer 393 will serve as anetch stop and an AR coating for a nitride DOE 394A (FIG. 10B). Therefractive index of oxide layer 393 is typically 1.45. The thickness ofoxide layer 393 is selected as described above (e.g., 990nm/4/1.45=170.7 nm). As nitride DOE 394A has a refractive index ofapproximately 2.1, oxide layer 393 with refractive index of 1.45 can bea very effective AR coating material.

In step 364, as shown in FIG. 10B, a lens layer 394 is formed to coveroxide layer 393 and then planarized. As can be seen, a portion of lenslayer 394 that conforms to the pattern of oxide layer 393 form a DOE394A. Lens layer 394 can be silicon nitride (Si₃N₄) or silicon dioxide(SiO₂) deposited by PECVD and is typically 1200 nm thick. Lens layer 394can be planarized by CMP to 900 nm thick.

In step 365, as shown in FIG. 10B, a transparent substrate 398 is bondedto etch mask layer 394. Transparent substrate 398 is transmissive to awavelength of interest (e.g., to a wavelength selected from infrared toultraviolet) if a percentage of the incident light specified by theapplication is transmitted through the substrate (e.g., at least 10% formost communication applications). Transparent substrate 34 can bequartz, sodium borosilicate glass (e.g., Pyrex®), sapphire, or fusesilica. If transparent substrate 398 is sodium borosilicate glass, itcan be bonded to a nitride lens layer 394 by an anodic bond.Alternatively, a silicon bonding layer can be formed on an oxide lenslayer 394 and a sodium borosilicate glass substrate 398 can be bonded tothe silicon bonding layer by an anodic bond.

In step 366, as shown in FIG. 10B, bonding pads 395 are formed on thebackside of silicon substrate 392. Bonding pads 395 can betitanium-platinum-gold (TiPtAu) sequence deposited by e-beam evaporationor sputtering and patterned by a liftoff mask. Titanium has a typicalthickness of 50 nm, platinum has a typical thickness of 150 nm, and goldhas a typical thickness of 50 nm.

In step 368, as shown in FIG. 10B, an etch mask layer 396 is formed overthe backside of silicon substrate 392 and bonding pads 395. Etch masklayer 396 can be silicon nitride (Si₃N₄) deposited by PECVD.

In step 372, as shown in FIGS. 10B and 10C, etch mask layer 396 ispatterned to form an etch window 400.

In step 374, as shown in FIGS. 10C and 10D, a portion of siliconsubstrate 392 located opposite of DOE 394A and exposed by etch window400 is etched down to oxide layer 393 to form a bonding ring 392A.Silicon substrate 392 can be etched away by DRIE.

In step 376, etch mask layer 396 is etched away. At this point, a lid402 is formed and can be mounted to a submount to complete amicroelectronic package.

Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention. Numerousembodiments are encompassed by the following claims.

1. A method for forming a diffractive lens, comprising: forming a stackcomprising at least two phase shifting layers separated by an etch stoplayer above a first surface of a transparent substrate, the transparentsubstrate being transmissive to a light wavelength selected frominfrared to ultraviolet; and patterning the stack to form layers of adiffractive optical element.
 2. The method of claim 1, wherein thetransparent substrate comprises a material selected from the groupconsisting of quartz, Pyrex, and sapphire.
 3. The method of claim 1,wherein said forming a stack comprises: (1) depositing a first phaseshifting layer comprising a material selected from the group consistingof amorphous silicon and silicon nitride; (2) growing an etch stop layercomprising silicon dioxide on the first phase shifting layer; and (3)depositing a second phase shifting layer comprising the material on theetch stop layer.
 4. The method of claim 1, further comprising forming anopaque coating on a second surface of the substrate.
 5. The method ofclaim 4, wherein the opaque coating comprises amorphous silicon.
 6. Themethod of claim 1, further comprising, prior to said forming a stack:forming an antireflective coating on the first surface of thetransparent substrate, wherein the stack is formed on the antireflectivecoating.
 7. The method of claim 1, further comprising, subsequent tosaid patterning the stack: forming an antireflective coating over thediffractive optical element.
 8. The method of claim 1, furthercomprising bonding a bonding ring to the first surface of thetransparent substrate around the diffractive optical element.
 9. Themethod of claim 8, wherein said bonding comprises forming a bond betweenthe bonding ring and the transparent substrate selected from the groupconsisting of an anodic bond, an adhesive bond, a hydrofluoric acidbond, and a glass frit bond.
 10. The method of claim 8, furthercomprising bonding a submount to the bonding ring to form a package. 11.The method of claim 1, further comprising bonding a submount to thefirst surface of the transparent substrate with silicone.
 12. The methodof claim 1, wherein the transparent substrate comprises a device layerof a silicon-on-insulator (SOI) substrate, the SOI substrate furthercomprising an insulator layer below the device layer and a handle layerbelow the insulator layer, the method further comprising: etching thehandle layer to the insulator layer to remove a portion of the handlelayer opposite the diffractive optical element, wherein the remainingportion of the handle layer forms a bonding ring.
 13. The method ofclaim 12, further comprising: etching the insulator layer to remove aportion of the insulator layer opposite the diffractive optical element.14. The method of claim 13, further comprising: forming anantireflective coating on a second surface of the device layer oppositethe diffractive optical element.
 15. The method of claim 12, furthercomprising: forming a bonding pad on the bonding ring.
 16. The method ofclaim 12, further comprising: forming a planarization layer over thediffractive optical element; and planarizing the planarization layer.17. The method of claim 16, further comprising: forming anantireflective layer on the planarization layer.
 18. A diffractive lens,comprising: a transparent substrate being transmissive to a lightwavelength selected from infrared to ultraviolet; and a diffractiveoptical element above a first surface of the transparent substrate, thediffractive optical element comprising at least two phase shiftinglayers separated by an etch stop layer.
 19. The lens of claim 18,wherein the transparent substrate comprises a material selected from thegroup consisting of quartz, Pyrex, and sapphire.
 20. The lens of claim18, further comprising an opaque coating on a second surface of thesubstrate.
 21. The lens of claim 20, wherein the opaque coatingcomprises amorphous silicon.
 22. The lens of claim 18, furthercomprising: an antireflective coating between the first surface of thetransparent substrate and the diffractive optical element.
 23. The lensof claim 18, further comprising: an antireflective coating over thediffractive optical element.
 24. The lens of claim 18, furthercomprising: a bonding ring bonded to the first surface of thetransparent substrate around the diffractive optical element.
 25. Thelens of claim 24, wherein the bonding ring is bonded to the transparentsubstrate by a bond selected from the group consisting of an anodicbond, an adhesive bond, a hydrofluoric acid bond, and a glass frit bond.26. The lens of claim 18, further comprising: a submount bonded to thebonding ring to form a package.
 27. The lens of claim 18, furthercomprising: a submount bonded to the first surface of the transparentsubstrate with silicone.
 28. The lens of claim 18, wherein thetransparent substrate comprises a device layer of a silicon-on-insulator(SOI) substrate, the SOI substrate further comprising an insulator layerbelow the device layer and a handle layer below the insulator layer, thehandle layer being etched so the remaining portion of the handle layerforms a bonding ring.
 29. The lens of claim 28, further comprising: anantireflective coating on a second surface of the device layer oppositethe diffractive optical element.
 30. The lens of claim 28, furthercomprising: a bonding pad on the bonding ring.
 31. The lens of claim 28,further comprising: a planarization layer over the diffractive opticalelement.
 32. The lens of claim 28, further comprising: an antireflectivelayer over the planarization layer.
 33. A method for forming adiffractive lens, comprising: forming an etch stop layer on a firstsurface of a silicon substrate; forming a diffractive optical elementabove the etch stop layer; forming a planarization layer over thediffractive optical element; planarizing the planarization layer;bonding a transparent substrate to the planarization layer, thetransparent substrate being transmissive to a light wavelength selectedfrom infrared to ultraviolet; and etching a second surface of thesilicon substrate to the etch stop layer to remove at least a portion ofthe silicon substrate opposite the diffractive optical element.
 34. Themethod of claim 33, wherein the transparent substrate comprises amaterial selected from the group consisting of quartz, Pyrex, andsapphire.
 35. The method of claim 33, wherein said forming a diffractiveoptical element comprises: forming a stack comprising at least two phaseshifting layers separated by another etch stop layer above; andpatterning the stack to form layers of the diffractive optical element.36. The method of claim 33, wherein said bonding a transparent substrateto the planarization layer comprises: forming a bonding layer on theplanarization layer; and bonding the transparent substrate on thebonding layer by an anodic bond.
 37. The method of claim 33, furthercomprising, prior to said forming a diffractive optical element: formingan antireflective layer on the etch stop layer, wherein the diffractiveoptical element is formed on the antireflective layer.
 38. The method ofclaim 37, further comprising: etching the etch stop layer to remove aportion of the etch stop layer opposite the diffractive optical element.39. The method of claim 33, wherein the remaining portion of the siliconsubstrate forms a bonding ring.
 40. The method of claim 39, furthercomprising: forming a bonding pad on the bonding ring.
 41. The method ofclaim 39, further comprising: bonding a submount to the bonding ring toform a package.
 42. The method of claim 33, wherein said etching asecond surface of the silicon substrate further comprises removing allof the silicon substrate.
 43. A diffractive lens, comprising: atransparent substrate being transmissive to a light wavelength selectedfrom infrared to ultraviolet; a planarization layer below thetransparent substrate; a diffractive optical element below theplanarization layer; and an etch stop layer below the diffractiveoptical element.
 44. The diffractive lens of claim 43, wherein thetransparent substrate comprises a material selected from the groupconsisting of quartz, Pyrex, and sapphire.
 45. The diffractive lens ofclaim 43, wherein the diffractive optical element comprises at least twophase shifting layers separated by another etch stop layer.
 46. Thediffractive lens of claim 43, further comprising: a bonding layerbetween the transparent substrate and the planarization layer.
 47. Thediffractive lens of claim 43, further comprising: an antireflectivelayer between the etch stop layer and the diffractive optical element.48. The diffractive lens of claim 43, further comprising: a bonding ringbelow the etch stop layer.
 49. The diffractive lens of claim 48, furthercomprising: a bonding pad on the bonding ring.
 50. The diffractive lensof claim 48, further comprising: a submount bonded to the bonding ringto form a package.
 51. A method for forming a diffractive lens,comprising: forming a mold for a diffractive optical element on a firstsurface of a silicon substrate; forming a lens layer above the mold,wherein the lens layer conforms to the mold to form the diffractiveoptical element, the lens layer being transmissive to a light wavelengthselected from infrared to ultraviolet; planarizing the lens layer;bonding a transparent substrate to the lens layer; and etching a secondsurface of the silicon substrate opposite of the diffractive opticalelement, wherein the remaining portion of the silicon substrate forms abonding ring.
 52. The method of claim 51, further comprising, prior tosaid forming a lens layer above the mold: forming an etch stop layer onthe mold; and wherein the lens layer is formed on the etch stop layerand said etching a second surface of the silicon substrate comprisesetching the silicon substrate to the etch stop layer.
 53. The method ofclaim 51, wherein the lens layer comprises a material selected from thegroup consisting of silicon nitride and silicon dioxide.
 54. The methodof claim 51, wherein the transparent substrate comprises a materialselected from the group consisting of quartz, Pyrex, and sapphire. 55.The method of claim 51, wherein said forming a mold comprises: forming astack comprising at least two lens layers separated by an etch stoplayer; and patterning the stack to form layers of the diffractiveoptical element.
 56. The method of claim 51, further comprising: forminga bonding pad on the bonding ring.
 57. The method of claim 51, furthercomprising bonding a submount to the bonding ring to form a package. 58.A diffractive lens, comprising: a transparent substrate beingtransmissive to a light wavelength selected from infrared toultraviolet; a diffractive optical element below the transparentsubstrate; and a bonding ring below the diffractive optical element. 59.The diffractive lens of claim 58, further comprising: an etch stop layerbetween the diffractive optical element and the bond ring.
 60. Thediffractive lens of claim 58, wherein the diffractive optical elementcomprises a material selected from the group consisting of siliconnitride and silicon dioxide.
 61. The diffractive lens of claim 58,wherein the transparent substrate comprises a material selected from thegroup consisting of quartz, Pyrex, and sapphire.
 62. The diffractivelens of claim 58, further comprising: a bonding pad on the bonding ring.63. The diffractive lens of claim 58, further comprising: a submountbonded to the bonding ring to form a package.